New York, An experimental dengue vaccine can fully protect healthy people who were intentionally infected with a weak form of the dengue virus in a human challenge test, says a study. Forty-one healthy individuals, who have not been previously exposed to dengue, received either a single shot of the vaccine called TV003 or a placebo. They were then infected six months later with a weakened form of dengue serotype two virus, the hardest serotype to prevent. While the 20 people in the placebo group developed mild symptoms like rash and low white blood cell count, all 21 vaccinated individuals developed none of these symptoms and were completely protected from infection. The vaccine was well-tolerated in all volunteers, said the study published in the journal Science Translational Medicine. While only advanced clinical trials can establish a vaccine's efficacy, human challenge studies could help identify promising vaccine candidates and eliminate poor ones from progressing to larger trials, the researchers said. "This work used a robust method which predicts a high likelihood of success for this critically important dengue vaccine," said study first author Beth Kirkpatrick, professor at University of Vermont (UVM) in the US. Human challenge model is already in use for studying malaria vaccines. "The dengue vaccine will now proceed to the ultimate test of effectiveness: large field-based trials in dengue-endemic countries," Kristen Pierce, associate professor of medicine at UVM, noted. "Beginning later this month, our team will be testing this vaccine in Dhaka, Bangladesh, and a large phase III efficacy trial has already begun in Brazil," Pierce said. A dengue vaccine that recently became available in Mexico, Brazil, the Philippines and El Salvador may not be suitable for all countries as it is beleived to be efective only in those who already had a dengue infection in their lives, and not in those who have not been exposed to dengue virus before. Dengue is the most common mosquito-borne viral infection in the world, infecting 390 million people annually. While the virus usually causes no symptoms or only mild fever, more than two million people each year develop life-threatening dengue shock syndrome.